Do physical damp proof courses fail?

Why DPC Injection Work is Rarely Required.

The damp proofing industry in the UK commonly promote two statements that are fundamental to this industry. Firstly, they promote rising damp as a common occurrence and we can comfortably state that this is simply untrue. It is an academically proven fact that rising damp is incredibly rare.

The second claim, which is also fundamental to an industry that sells retrofit chemical injection and re-plastering is that physical damp proof courses commonly fail. We have reviewed many many reports from these ‘specialist’ companies and the absence or failure of an existing physical DPC is commonly cited as justification for installing a retrofit chemical injection system. Moreover, you have all commonly seen retrofit chemical injection work installed where physical DPC’s already exist.

Do Damp Proof Courses Fail?

Bitumen felt DPC

Originally pointed over but extrusion has blown the mortar. This DPC has not failed.

Very old slate DPC

Fully functional slate DPC but bridged by rainsplash due to high ground levels

Hidden slate DPC

Physical DPC simply bridged by soil banked against the wall. Guess what the solution is?

Functional Slate DPC

DPC still fully functional despite being bridged by OPC mortar at the bed joint.

Visibly Functional Slate DPC

Despite localised flooding due to a blocked gulley.

New Polyethylene DPC

New plastic DPC's are commonly bridged by poorly informed builders

Correctly installed Polyethylene DPC

DPC installed with the required overlap to prevent bridging at the bed joint

Damp proofers

Why let the presence of a functional DPC get in the way of selling you another one.

More pointless injection work

Injection work of this sort is inappropriate for old properties and nothing short of vandalism.

No physical damp proof course present

Not a problem in this windmill provided the wall base had been allowed to breathe.

Cracked rainwater gulley

The lack of a physical DPC need not be a problem if local ground moisture is managed.

High ground levels and blocked gulley

The solution to dealing with wall base damp very rarely needs 'specialist' treatments.

No DPC present & high external ground levels

A problem that was cured by reinstating critical technical details.

Bitumen DPC

Extruded from wall but fully functional

There are of course legislative requirements for the insertion of a physical dpc in new buildings. Approved document C, Section 5.2, states that walls should: resist the passage of moisture from the ground to the inside of the building; and not be damaged by moisture from the ground to any part which would be damaged by it. This requirement is met if a damp proof course is provided of; bituminous material, polyethylene, engineering bricks or slates in cement mortar or any other material that will prevent the passage of moisture. However, relatively speaking this is modern requirement and we have many thousands of properties in the UK that do not have have a physical damp proof course installed and yet they manage moisture perfectly well despite non-compliance with the modern requirement for a physical DPC.

I personally carried out a comprehensive review of this very question and what became clear is that the majority of academic commentary cited bridging rather than failure as the key issue, in fact it is fair to say that there was general agreement on this point. We found only two cases where commentators cited their view that DPC’s fail, in both cases these were unproven opinion rather than proven fact. Here is an opinion given by Trotman P, Sanders C, Harrison H (2004)… Physical dpc’s can fail occasionally, particularly those formed by engineering bricks or overlapping slates, following breakdown of the mortar; bitumen felt dpc’s can become brittle with age. The ‘breakdown of mortar’ is the most interesting point in this statement but the idea that an engineering brick can fail is simply wrong. The authors do not go on to explain their point but we can only assume that this idea is linked to occasional building movement that results in cracked engineering bricks at DPC level. A crack in a brick or a slate DPC will not result in capillary rise in those units and we are firmly of the opinion that engineering brick DPC’s do not fail. Moreover they are the simplest physical DPC to visually inspect. The key controversy must focus on hidden DPC’s installed to the mortar bed joint. These can be formed from a wide range of materials including poured bitumen, bitumen felt, lead, copper, overlapping slates and probably one or two more that currently escape my mind. They are often not even visible at the bed joint and this may be due to being hidden by high external ground levels, or more commonly, they have been pointed over. Both issues are clearly bridging issues rather than DPC failure and if you have a bridge then the simple solution to that problem is to remove the bridge.

To my knowledge no one has carried out a piece of research into alleged DPC failures and published their findings. It can’t be done by the damp proofing industry because they have a vested interest in promoting the idea of DPC failure. It would need to be an independent piece of work that to my mind would be a valuable piece of research. I have considered co-ordinating this with a demolition company so that every time a building is taken down we can thoroughly inspect the DPC in the process. We have removed bricks from walls on many many occasions to inspect cavities and where we do this we have consistently found the old physical DPC to be intact and fully functional.

We have previously written that Portland cement degrades over time, initially it is resistant to rising damp until after many years of degradation it then becomes the major moisture pathway for rising damp. Where a continuous physical barrier is installed then clearly this is not a problem but this fact may well form at least a partially valid argument towards a claim that an engineering brick DPC has failed. Technically there would be nothing wrong with bricks but the mortar perps may allow rising damp via diffusion. Interestingly we have seen where perp joints have been left open on engineering brick DPCs and this would completely mitigate for this potential issue. However, in all alleged cases of DPC failure, what we commonly recommend is that so long as there is a provision for adequate wall base ventilation then this does not become an issue. It is all about maintaining moisture equilibrium, which is ensuring that moisture is evaporating off the wall as fast as it is rising. Similarly, where we find that physical DPC’s are hidden we simply treat the building as though a physical DPC is not installed so that if external finished floor levels are a minimum of 200mm below internal finished floor level then this need not be a problem. There are thousands of properties in this country that perform perfectly well without a physical DPC and they generally do so because moisture equilibrium is maintained in their walls due to the fact that they are left bare, they are correctly repointed with lime mortar, there is adequate subfloor ventilation, external finished floor levels are not too high and local ground moisture is managed. You can of course apply all or most of these principles to a building that has a physical DPC installed, even one that has allegedly failed and you would mitigate for the alleged failure.

We are lucky enough to carry out a great deal of survey work on the Crown Estate. We deal with some very old historic buildings that were originally built to a very high standard. We are seeing properties over 150 years old where ordinarily we would not expect to see a physical DPC installed but on this Estate they do, and this gives us a rare insight into some quite unique properties. Many of the images contained within this blog are from the Crown Estate and we are consistently finding perfectly functional DPC’s in some of the oldest properties to have physical DPC’s installed. I may not have proven through this blog that physical DPC’s don’t fail but I can state with certainty that no one has proved that they do. We do not believe that physical DPC’s fail so if one is installed then you should give careful thought as to why you would even consider installing another unproven retrofit chemical injection system in the absence of any proof that the existing physical system has failed. We have always taken a balanced view on retrofit DPC injection because pragmatically there are times when lowering external ground levels may not be an option but the fact remains that we very rarely have a need to specify these management solutions because our focus is always on curing rather than managing or hiding the problem.

One comment

I’ve got a garage which has what seems to be a bitumen felt DPC like the one shown in your third photo. Originally there was pointing over the DPC but over the last couple of years this has come away so the edge of the DPC is now clearly visible between the two courses of bricks all the way round the building. So it looks like the first photo in your carousel of images (where you say extrusion has blown the mortar) except that in my case the pointing covering the DPC has come away pretty much everywhere rather than just in a few places.

Will that potentially give me a problem with damp? Do I need to rush to get it repointed or is this nothing more than a cosmetic issue?